It is well known to use electromechanical slip rings, to enable the transfer of data to and from a rotating device, such as the drum that is rotatably mounted in the gantry of a computer tomography (CT) scanner so that the X-ray equipment supported by the drum can be rotated about a patient during a tomographic scan. It is equally well known to employ a radio frequency (RF) link to enable such a data transfer. For example, U.S. Pat. No. 5,577,026, entitled "Apparatus for Transferring Data to and from a Moving Device," illustrates the use of an RF link to transfer data in a CT scanner.
U.S. Pat. Nos. 4,928,283 and RE 34,379 briefly describe a CT scan system employing a "two-way communication link" between "an electronics package" secured to the rotating disk of the system and a "computer used for image processing and control". No further detail is provided.
In a CT scanning system, the two-way communication link (hereinafter data link) typically has a maximum throughput rate that is not easily increased due to physical limitations of the components used to implement the data link. Thus, the number of views of raw data per rotation of the CT scanner is limited by the data transfer rate over the data link. However, the quality of the reconstructed image can be improved by using more views of data. For example, an existing CT system can collect and transfer up to 960 views per rotation. It has been shown that using 1440 to 1920 views per rotation significantly improves the reconstructed image, for example by reducing blurriness around the edges of the image. Increasing the number of views to be processed in existing systems is often difficult or impossible due to limitations of the data link throughput. One method of increasing the effective throughput rate of the data link without modifying the existing hardware components and configurations used to implement the data link is to utilize data compression techniques prior to transferring the raw data. However, conventional data compression techniques are generally not suitable for compressing raw CT data. First, the compression rate of a conventional compression algorithm is generally dependent upon the redundancy of the bit pattern in the data being compressed, i.e., the more repetition of the constituent bit patterns, the more the data will compress without the subsequent loss of data. Raw CT data tends to be highly random in the bit pattern, so the compression rate necessarily has to be low in order to preserve the data. As a result the available compression rate using conventional compression techniques are typically inadequate for CT applications. Second, the compression rates provided by conventional compression techniques are data dependent, and thus variable, which makes the transfer protocol more complex and the error checking more difficult in a CT application. Third, rare but relatively large loss in the compressed data can possibly cause destructive damage to the reconstructed CT image. Many conventional compression techniques were designed for applications that do not require that a constant data compression rate be maintained, or for applications that can tolerate an occasional large loss of data.
It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art, and to provide a data compression technique suitable for raw CT data.